1. Field of the Invention
The present invention relates to a process for preparing isocyanates in high yield by the thermal decomposition of carbamates in the presence of a catalyst under gaseous phase condition. Isocyanates which have a superior reactivity are useful compounds that are extensively used as a raw material for urethanes, pharmaceuticals, pesticides, etc.
2. Description of the Prior Art
In the industrial field, isocyanates are prepared by reacting amines with phosgene. However, this process requires strongly toxic phosgene. Moreover, a large amount of hydrogen chloride by-products are produced, which corrode a reactor. Therefore, processes for preparing isocyanates without requiring phosgene are strongly demanded.
Among the processes, those by thermal decomposition of carbamates are most commonly proposed, which are roughly classified into two groups: processes carried out in the presence of a catalyst under liquid phase condition, and processes carried out in the presence of a catalyst under high temperature gaseous phase condition.
Examples of the disclosed processes by liquid phase reaction are as listed below: a process wherein a basic catalyst such as alkaline-earth metal and its inorganic compounds is used (Laid-open Japanese Patent application No.54-88201); a process wherein a heavy metal catalyst or heavy metal compounds catalyst is used in an inert solvent (Laid-open Japanese Patent application No.51-19721); a process wherein a metal compound catalyst of at least one element selected from the group consisting of Ib group, IIb group, IIIa group, IVa group, IVb group, Vb group, and VIII group is used in an inert solvent (Laid-open Japanese Patent application No.56-166160); a process wherein zinc chloride is used as a catalyst (Laid-open Japanese Patent application No.57-21356); a process wherein a compound of Tl, Sn, Sb, or Zr is used as a catalyst (Laid-open Japanese Patent application No.58-128354); a process carried out under reduced pressure wherein a metal simple substance or a metal compound of Mn, Mo, W, or Zn is used in a solvent with a high boiling point (Laid-open Japanese Patent application No.2-134355). However, since the catalysts used in these processes are homogeneously dissolved in a reaction solvent, a large amount of energy is required for separating out the catalyst from the decomposition products, and components of the catalyst may be included in the products. Moreover, when the above processes are adopted, the catalyst used in the reaction solvent is difficult to be recovered and re-utilized, and thus the catalyst must be treated as waste product.
For the effective processes to solve the above problems, the processes, wherein a solid solvent which is not dissolved in the reaction solvent under the reaction condition is used, have been proposed.
The disclosed processes are as listed below: a process wherein metal zinc, aluminum, titanium, iron, chromium, cobalt, or nickel is used in the form of enlarged surface area as a catalyst (Laid-open Japanese Patent application No.56-65857); a process wherein one element selected from the group consisting of copper group, zinc group, aluminum group, carbon group (excluding carbon), and titanium group is used in the form of simple substance, oxide, or sulfide (Laid-open Japanese Patent application No.57-158747); a process wherein one element selected from the group consisting of carbon group (excluding carbon), titanium group, vanadium group, and chromium group is used as a catalyst in the form of carbide or nitride (Laid-open Japanese Patent application No.57-158748), and a method wherein a catalyst including boron is adopted (Laid-open Japanese Patent application No.63-211256). However, in the above processes, the catalytic activities are low, and relatively large amounts of catalysts are required. Thus, a yield of isocyanates which is high enough for industrial use cannot be obtained through these processes. Moreover, a large amount of energy is required for separating out and recovering the solvent.
For the processes by gaseous phase reaction, for example, a process wherein thermal decomposition in a gaseous phase is carried out at a temperature in the range of 400.degree. C.-600.degree. C. in the presence of a Lewis acid catalyst (Laid-open Japanese Patent application No.46-17773) is disclosed. However, a large amount of polymeric by-products are produced through the process. Moreover, the thermal decomposition of the catalyst makes the life thereof shorter, and the reactor is corroded. The process also has the problem that the yield of the desired products is low. Another method is also disclosed wherein a reaction in a gaseous phase is carried out under reduced pressure in the presence of a gas permeant packing having a heat resisting property consisting of steel, brass, copper, zinc, aluminum, titanium, chromium, cobalt, nickel, carbon, or quartz (Laid-open Japanese Patent applications No.59-205352, No. 59-205353). The process also has the problem that the yield of the desired product is low. Moreover, since the reaction is carried out under reduced pressure at high temperature of 410.degree. C., high cost is required for the reactor.
As described, the processes for preparing isocyanates wherein solid acid or solid base is used as a catalyst for the thermal decomposition of carbamates under gaseous phase condition have been proposed. However, all of the above processes have the problems of low yield, and a large amount of polymeric by-products or urea compounds. Therefore, the catalyst for gaseous phase reaction for industrial use has not yet been introduced.